Known circuits have often included means sensitive to the speed of the motor in the form of a transducer mechanically driven by the motor (or by the vehicle transmission). Generally speaking the transducer produces an a.c. signal which varies in frequency with the speed of the motor. Such a transducer is, however, considered to be an undesirably expensive component of the control system and it is an object of the invention to provide a control system of the general kind referred to in which no mechanical speed transducer is utilized.